The plates that make up the earth's lithosphere are constantly in motion.
The rate of motion is a
few centimetres per year, or approximately 0.1 mm per day (about as fast as your fingernails
grow). This does not mean, however, that the rocks present at the places where plates meet (e.g.,
convergent boundaries and transform faults) are constantly sliding past each other. Under some
circumstances they do, but in most cases, particularly in the upper part of the crust, the friction
between rocks at a boundary is great enough so that the two plates are locked together. As the
plates themselves continue to move, deformation takes place in the rocks close to the locked
boundary and strain builds up in the deformed rocks. This strain, or
represents potential energy stored within the rocks in the vicinity of the boundary between two
plates. Eventually the strain will become so great that the friction and rock-strength that is
preventing movement between the plates will be overcome, the rocks will break and the plates
will suddenly slide past each other - producing an earthquake [see Fig. 10.4].
A huge amount of energy will suddenly be released, and will radiate away from the location of
the earthquake in the form of deformation waves within the surrounding rock.
(compression waves) are known as body waves as they travel through the
As soon as this happens, much
of the strain that had built up along the fault zone will be
Earthquakes occur in three main situations:
Earthquakes at spreading ridges
tend to be frequent but small
because the rock is too warm to
allow significant locking.
Earthquakes along transform faults
are less frequent, but they can be
quite large. Most transform faults
are beneath the oceans, but some
are on land.
The deeper parts of subduction
zones can have many small
earthquakes, but in the shallower
parts, where the rocks are cool,
earthquakes are less frequent and
can be very large.
The severity of earthquakes generated in any area will depend
primarily on the degree to which the two plates are locked
together along the boundary zone. The degree of locking will
depend on several factors, including the rock types, the level of
water saturation along the boundary, and the temperature.
Not all of the strain is necessarily released with the first movement on the rock - especially for large earthquakes.
Additional movement - which produces aftershocks - can take place for several days and weeks, and in some cases
for months and years.